Sampled moving average notch filter for ripple reduction in chopper stabilized operational amplifiers

ABSTRACT

A chopper-stabilized amplifier includes a first transconductance amplifier and a first chopper circuit coupled to an input of the first transconductance amplifier. A second chopper circuit is coupled to an output of the first transconductance amplifier. The chopper-stabilized amplifier also includes second and third transconductance amplifiers having inputs coupled to the output of the first transconductance amplifier. The second transconductance amplifier produces an output responsive to a first notch clock signal having a first phase relative to the chopping of the second chopper circuit. The third transconductance amplifier produces an output responsive to a second notch clock signal having a second phase relative to the first phase. The output signals produced by the second and third transconductance amplifiers are added to filter ripple noise at the outputs of the second and third transconductance amplifiers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Indian provisional application No.201841038728, filed Oct. 12, 2018, entitled “CHOPPER RIPPLE ELIMINATIONUSING SAMPLED MOVING AVERAGE NOTCH FILTER”, assigned to the presentassignee and incorporated herein by reference.

BACKGROUND

The invention relates generally to reducing ripple noise in chopperstabilized operational amplifier circuits, and more particularly tosampled moving average notch filters for ripple noise reduction inchopper stabilized operational amplifier circuits.

DESCRIPTION OF THE RELATED ART

Chopper stabilization is used to reduce offset voltage, offset voltagedrift and flicker noise in amplifiers. Chopper stabilized amplifiersmaintain the noise charactersitics of their input stages, but shifttheir input offset voltages to the chopping frequency, creating largeripple noise at the amplifier outputs.

FIG. 1 illustrates a three-stage amplifier 100 with chopperstabilization circuitry. This circuit configuration includes athree-stage high gain signal path including three sequentially coupledoperational transconductance amplifiers 104, 108 and 112 havingtransconductances of gm1, gm2, and gm3, respectively. The three-stagehigh gain signal path is coupled in parallel with a wider bandwidth twostage signal path including two sequentially coupled operationaltransconductance amplifiers 116 and 112 having transconductances of gm4and gm3, respectively. A first chopper stabilization circuit 120including switches S1, S2, S3, and S4 is added before stage 2 (i.e.,input stage), and a second chopper stabilization circuit 124 includingswitches S5, S6, S7, and S8 is added after stage 2. The chopperstabilization reduces offset voltage, offset voltage drift, and flickernoise, but shifts the offset voltage of the input stage to the choppingfrequency and thereby producing a large ripple noise in the amplifieroutput Vout.

In order to reduce ripple in the amplifier output, switched capacitornotch filters have been incorporated into amplifiers. FIG. 2Aillustrates a three stage amplifier 200 including a switched capacitornotch filter circuit 204. The amplifier 200 is similar to the amplifier100 except for the addition of the switched capacitor notch filtercircuit 204 to the high gain signal path at the output of the secondchopper stabilization circuit 124. The notch filter circuit 204 includesa sample and hold circuit controlled by a notch clock signal Fnotch. Theswitched capacitor notch filter operates at 90 degrees out of phase witha chopper clock Fs to filter out the ripple at the chopper frequency.FIG. 2B illustrates the ripple voltage Vripple, the chopper clock Fs andthe notch clock signal Fnotch.

As shown in FIG. 2B, if the switched capacitor notch filter 204 isoperated at precisely 90 degrees out of phase with the chopper clock Fs,ripple will be eliminated. If, however, there is a skew between thechopper clock Fs and the notch clock Fnotch, there remains a residualripple. Thus, if the switched capacitor notch filter 204 is not operatedat exactly 90 degrees out of phase with the chopper circuits, thereremains a residual ripple Vripple as shown in FIG. 2C.

What is needed is a chopper-stabilized amplifier that does not requirethe generation of control signal that is precisely 90 degrees out ofphase with the chopper clock and is tolerant of clock skew between thechopper clock and the notch filter clock.

SUMMARY

In accordance with one embodiment of the invention, a chopper-stabilizedamplifier includes a first transconductance amplifier and a firstchopper circuit coupled to an input of the first transconductanceamplifier. The first chopper circuit is configured to chop an inputsignal to apply the chopped input signal to the input of the firsttransconductance amplifier. A second chopper circuit is coupled to anoutput of the first transconductance amplifier. The second choppercircuit is configured to chop an output of the first transconductanceamplifier to produce a second chopped signal. A first and a secondcapacitor are coupled between an output of the second chopper circuitand ground. The chopper-stabilized amplifier also includes a secondtransconductance amplifier having an input coupled to receive the secondchopped signal. The second transconductance amplifier produces an outputresponsive to a first notch clock signal having a first phase relativeto the chopping of the second chopper circuit. The chopper-stabilizedamplifier also includes a third transconductance amplifier having aninput coupled to receive the second chopped signal. The thirdtransconductance amplifier produces an output responsive to a secondnotch clock signal having a second phase relative to the first phase.The output signals produced by the second and third transconductanceamplifiers are added to filter ripple voltages at the outputs of thesecond and third transconductance amplifiers. The chopper-stabilizedamplifier also includes a fourth transconductance amplifier having aninput coupled to receive the outputs of the second and thirdtransconductance amplifiers. A compensation capacitor is coupled betweenthe input and output of the fourth transconductance amplifier. Thechopper-stabilized amplifier also includes a fifth transconductanceamplifier having an input coupled to receive the input signal and anoutput coupled to the input of the fourth transconductance amplifier.

In accordance with another embodiment, a chopper-stabilized amplifierincludes a first transconductance amplifier and a first chopper circuitcoupled to an input of the first transconductance amplifier. The firstchopper circuit is configured to chop an input signal to apply thechopped input signal to the input of the first transconductanceamplifier. A second chopper circuit is coupled to an output of the firsttransconductance amplifier. The second chopper circuit is configured tochop an output of the first transconductance amplifier to produce asecond chopped signal. A first and a second capacitor are coupledbetween an output of the second chopper circuit and ground. A sampledmoving average notch filter having differential inputs is coupled toreceive the second chopped signal. The sampled moving average notchfitler produces a first output responsive to a to a first notch clocksignal having a first phase relative to the chopping of the secondchopper circuit and a second output responsive to a second notch clocksignal having a second phase relative to the first phase. The first andsecond outputs are added to filter ripple voltages at the outputs of thesampled moving average notch filter. The sampled moving average filterincludes a second transconductance amplifier having differential inputscoupled to receive the second chopped signal and is operable to producethe first output responsive to the first notch clock signal. The sampledmoving average notch filter includes a third transconductance amplifierhaving differential inputs coupled to receive the second chopped signaland is operable to produce the second output responsive to a secondnotch clock signal. The chopper-stabilized amplifier also includes afourth transconductance amplifier having an input coupled to receive theoutputs of the second and third transconductance amplifiers and acompensation capacitor coupled between the input and output of thefourth transconductance amplifier. The chopper-stabilized amplifier alsoincludes a fifth transconductance amplifier having an input coupled toreceive the input signal and an output coupled to the input of thefourth transconductance amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a three-stage amplifier with chopper stabilizationcircuitry.

FIG. 2A illustrates a three stage amplifier including a switchedcapacitor notch filter circuit, and 2B and 2C show waveforms of signalsproduced by the amplifier.

FIG. 3A illustrates a chopper-stabilized amplifier in accordance withthe present disclosure.

FIG. 3B is a timing diagram for the chopper-stabilized amplifier of FIG.3A, and FIGS. 3C and 3D show waveforms of signals produced by theamplifier.

DETAILED DESCRIPTION

FIG. 3 A illustrates a chopper-stabilized amplifier 300 in accordancewith the present disclosure. The amplifier 300 includes a first choppercircuit 304 between differential input voltages Vin− and Vin+ and thedifferential inputs of a first transconductance amplifier 308 having atransconductance gm1. The first chopper circuit 304 includes switchesS1, S2, S3 and S4 which are controlled by phase 1 and phase 2 signalsshown in FIG. 3B. Switches S1 and S4 are closed when phase 1 is high,and switches S2 and S3 are closed when phase 2 is high. The switches maybe MOSFETs or other types of switches. The first chopper circuit 304produces chopped input signals which are applied to the differentialinputs of the first transconductance amplifier 308.

The amplifier 300 includes a second chopper circuit 312 connected to thedifferential outputs of the transconductance amplifier 308. The secondchopper circuit 312 includes switches S5, S6, S7 and S8 which are alsocontrolled by the phase 1 and phase 2 signals shown in FIG. 3B. SwitchesS5 and S8 are closed when phase 1 is high, and switches S6 and S7 areclosed when phase 2 is high. The second chopper circuit 312 producessecond chopped signals at the differential outputs of the second choppercircuit 312. The first and second chopper circuits 304 and 312 areoperated synchronously at a chopping frequency Fs.

Capacitors 316 and 320 are coupled between the differential outputs ofthe second chopper circuit 312 and ground. Due to the chopping action,the output current of the second chopper circuit 312 has a squarewaveform which is integrated by the capacitors 316 and 320. As a result,triangular waveform voltages are generated across the capacitors 316 and320.

The amplifier 300 includes a second transconductance amplifier 324having a transconductance gm2. The second transconductance amplifier 324includes differential inputs coupled to receive the second choppedsignal. The second transconductance amplifier 324 also receives a clocksignal F1. The second transconductance amplifier 324 produces an outputsignal responsive to the clock signal F1. The clock signal F1 has afirst phase relative to the chopping of the chopper circuits. In someembodiments, F1 is in phase with Fs.

The amplifier 300 includes a third transconductance amplifier 328 havinga transconductance gm3. According to some disclosed embodiments, gm2 isequal to gm3. The third transconductance amplifier 328 includesdifferential inputs coupled to receive the second chopped signal. Thethird transconductance amplifier 328 also receives a clock signal F2.The third transconductance amplifier 328 produces an output signalresponsive to the clock signal F2. The clock signal F2 has a secondphase relative to the chopping of the chopper circuits. In someembodiments, F2 is 180 degrees out of phase with Fs.

Since F1 and F2 are 180 degrees out of phase with each other, the inputsignals applied to the transconductance amplifiers 324 and 328 have samevalue in the rising direction and the falling direction (i.e., equalmagnitude but opposite signs). Thus, ripple voltages in the inputs ofthe transconductance amplifiers 324 and 328 have the same positive andnegative values. The output signals produced by the second and thirdtransconductance amplifiers 324 and 328 are added at junction 326 tocancel ripple voltages. As discussed before, gm2 is equal to gm3, and asa consequence the ripple is canceled.

The amplifier 300 includes a fourth transconductance amplifier 332having a transconductance gm4. The fourth transconductance amplifier 332includes an input coupled to receive the outputs of the second and thirdtransconductance amplifiers 324 and 328. A miller compensation capacitor336 is coupled between the input and output of the fourthtransconductance amplifier 332. The transconductance amplifier 332produces an output voltage Vout.

The amplifier 300 includes a fifth transconductance amplifier 340 havinga transconductance gm5. The fifth transconductance amplifier 340includes differential inputs coupled to receive the input signals and anoutput coupled to the input of the fourth transconductance amplifier336. Thus, the amplifier 300 provides a high gain signal path includingfour transconductance amplifiers 308, 324, 328 and 336 havingtransconductances of gm1, gm2, gm3, and g4 respectively. The high gainsignal path is coupled in parallel with a wider bandwidth signal pathincluding two sequentially coupled transconductance amplifiers 340 and336 having transconductances of gm5 and gm4, respectively.

FIG. 3C illustrate ripple voltage at the output of the second choppercircuit 312, clock signals Fs, F1, F2, and residual ripple. As shown inFIG. 3C, a sample and hold function from one point (e.g., sample 1) tothe next point (e.g., sample 2) will result in a consistent output,suppressing the ripple when the outputs of the second and thirdtransconductance amplifiers 324 and 328 are added. Thus, even if thereis a skew between Fs and F1 or F2, the error sampled by the secondtransconductance amplifier 324 is canceled by the error sampled by thethird transconductance amplifier 328. Thus, the amplifier 300 istolerant towards clock skew between the chopping clock and the notchfilter clock. This is advantageous because in high frequency choppingclock skew errors become significant compared to low frequency chopping.

In one aspect, the second and third transconductance amplifiers 324 and328 having their outputs connected at the junction 326 operate as asampled moving average notch filter 344. The sampled moving averagenotch filter 344 is formed by sampling the in phase component by thetransconductance amplifier 324 and the out of phase component by thetransconductance amplifier 328, and adding the outputs of thetransconductance amplfiers 324 and 328 at the junction 326. The zeroesFz of the sampled moving average notch filter 344 may be expressed bythe relationship Fz=nFs. Hence, the sampled moving average notch filter344 produces notches at multiples of the chopping frequency Fs. Sincethe samples are acquired in-phase and out of phase relative to thechopping clock signal Fs, F1 and F2 can be generated using Fs. A higherclock frequency allows the implementation of the sampled moving averagenotch filter 344 on a smaller area on an integrated circuit.

The skew between Fs and F1 should be same as the skew between Fs and F2for the error to cancel. If the skew between Fs and F1 is different thanthe skew between Fs and F2, a DC offset voltage is produced at theoutput of the notch filter 344 which is turn in attenuated by the gainof the transconductance amplifier 308.

FIG. 3D illustrates performance of the amplifier 300. As shown in FIG.3D, the ripple voltage is approximately 1 μV regardless of a skewbetween Fs and F1 or F2. Even in the presence of offset voltages, thesecond and third transconductance amplifiers 324 and 328 functioning asa moving average sample and hold notch filter cancel the ripple.

In some embodiments, the amplifier 300 can be implemented in anintegrated circuit (IC) or in an application-specific integrated circuit(ASIC) system-on-a-chip (SoC). In other embodiments, the amplifier 300can be implemented with discrete components.

Various illustrative components, blocks, modules, circuits, and stepshave been described above in general terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. The described functionality may be implemented invarying ways for each particular application, but such implementationdecision should not be interpreted as causing a departure from the scopeof the present disclosure.

For simplicity and clarity, the full structure and operation of allsystems suitable for use with the present disclosure is not beingdepicted or described herein. Instead, only so much of a system as isunique to the present disclosure or necessary for an understanding ofthe present disclosure is depicted and described.

What is claimed is:
 1. A chopper-stabilized amplifier comprising: afirst transconductance amplifier; a first chopper circuit coupled to aninput of the first transconductance amplifier, the first chopper circuitconfigured to chop an input signal to apply the chopped input signal tothe input of the first transconductance amplifier; a second choppercircuit coupled to an output of the first transconductance amplifier andconfigured to chop an output of the first transconductance amplifier toproduce a second chopped signal; first and second capacitors coupledbetween an output of the second chopper circuit and ground; a secondtransconductance amplifier having an input coupled to receive the secondchopped signal and operable to produce an output responsive to a firstnotch clock signal having a first phase relative to the chopping of thesecond chopper circuit; and a third transconductance amplifier having aninput coupled to receive the second chopped signal and operable toproduce an output responsive to a second notch clock signal having asecond phase relative to the first phase, wherein output signalsproduced by the second and third transconductance amplifiers are addedto filter ripple noise at the outputs of the second and thirdtransconductance amplifiers.
 2. The chopper-stabilized amplifier ofclaim 1, wherein the second phase is 180 degrees out of phase with thefirst phase.
 3. The chopper-stabilized amplifier of claim 1, wherein thefirst and second chopper circuits are operated synchronously.
 4. Thechopper-stabilized amplifier of claim 1 further comprising: a fourthtransconductance amplifier having an input coupled to receive theoutputs of the second and third transconductance amplifiers; and acompensation capacitor coupled between the input and output of thefourth transconductance amplifier.
 5. The chopper-stabilized amplifierof claim 1 further comprising a fifth transconductance amplifier havingan input coupled to receive the input signal and an output coupled tothe input of the fourth transconductance amplifier.
 6. Achopper-stabilized amplifier comprising: a first transconductanceamplifier; a first chopper circuit coupled to an input of the firsttransconductance amplifier, the first chopper circuit configured to chopan input signal to apply the chopped input signal to the input of thefirst transconductance amplifier; a second chopper circuit coupled to anoutput of the first transconductance amplifier and configured to chop anoutput of the first transconductance amplifier to produce a secondchopped signal; first and second capacitors coupled between an output ofthe second chopper circuit and ground; a sampled moving average notchfilter having differential inputs coupled to receive the second choppedsignal and operable to produce a first output responsive to a to a firstnotch clock signal having a first phase relative to the chopping of thesecond chopper circuit and a second output responsive to a second notchclock signal having a second phase relative to the first phase, whereinthe first and second outputs are added to filter ripple voltages at theoutputs of the sampled moving average notch filter.
 7. Thechopper-stabilized amplifier of claim 6, wherein the sampled movingaverage notch filter further comprises: a second transconductanceamplifier having differential inputs coupled to receive the secondchopped signal and operable to produce the first output responsive tothe first notch clock signal; and a third transconductance amplifierhaving differential inputs coupled to receive the second chopped signaland operable to produce the second output responsive to a second notchclock signal.
 8. The chopper-stabilized amplifier of claim 7, whereinthe second phase is 180 degrees out of phase with the first phase. 9.The chopper-stabilized amplifier of claim 7, wherein the first andsecond chopper circuits are operated synchronously.
 10. Thechopper-stabilized amplifier of claim 7 further comprising: a fourthtransconductance amplifier having an input coupled to receive theoutputs of the second and third transconductance amplifiers; and acompensation capacitor coupled between the input and output of thefourth transconductance amplifier.
 11. The chopper-stabilized amplifierof claim 7 further comprising: a fifth transconductance amplifier havingan input coupled to receive the input signal and an output coupled tothe input of the fourth transconductance amplifier.
 12. Achopper-stabilized amplifier comprising: a first transconductanceamplifier; a first chopper circuit coupled to an input of the firsttransconductance amplifier, the first chopper circuit coupled to receivean input signal and configured to chop the input signal to apply thechopped input signal to the input of the first transconductanceamplifier; a second chopper circuit coupled to receive an output signalfrom the first transconductance amplifier and configured to chop theoutput signal to produce a second chopped signal; first and secondcapacitors coupled between an output of the second chopper circuit andground; a second transconductance amplifier having an input coupled toreceive the second chopped signal and operable to produce an outputresponsive to a first notch clock signal having a zero degree phaserelative to the chopping of the second chopper circuit; and a thirdtransconductance amplifier having an input coupled to receive the secondchopped signal and operable to produce an output responsive to a secondnotch clock signal having 180 degrees phase relative to the first notchclock signal, wherein output signals produced by the second and thirdtransconductance amplifiers are added to filter ripple voltages at theoutputs of the second and third transconductance amplifiers; a fourthtransconductance amplifier having an input coupled to receive theoutputs of the second and third transconductance amplifiers; acompensation capacitor coupled between the input and output of thefourth transconductance amplifier; and a fifth transconductanceamplifier having an input coupled to receive the input signal and anoutput coupled to the input of the fourth transconductance amplifier.13. The chopper-stabilized amplifier of claim 12, wherein the first andsecond chopper circuits are operated synchronously.